F.3 Pjysics {Substance 同 Matter 有咩分別 ?}

In chemistry, a distinction is often made between the use of the terms matter and substance. The term matter is used as a label according to one of the broadest criteria possible: whether an item possesses mass while it is stationary. If an item has mass when it is stationary, then it is part of the matter of the universe. Otherwise, it is classified as energy. At the level of subatomic particles, an example of matter is the electron, and an example of energy is the photon.
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We now consider a narrower criterion. Consider the myriad of chemical properties that any large amount of matter can possess. The term substance is used as a label for a collection of matter with specific chemical properties. The smallest unit of a substance is an atom. The largest unit of a substance is a molecule.
2

Consider the sentence Matter can exist in one of three states: a solid, a liquid, or a gas. This statement is meant to indicate that all of the matter in the universe exists in one of three states. The focus is not on specific chemical properties unique to some molecules or atoms. On the other hand, consider the statement Air is composed of the substances nitrogen, oxygen, argon, carbon dioxide, neon, and helium. This statement illustrates two points; first, air is a mixture of substances and is not considered a substance itself; second, the chemical properties of nitrogen are distinct from those of oxygen, argon, carbon dioxide, neon, and helium.
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Matter is a term that classifies everything in the universe according to mass. Substance is a term that classifies objects according to specific atomic and molecular properties. Between these two levels of classification, there is the vernacular classification we make use of every day—when we speak of metals, paper, and ink, for example. At this level of classification, we are not especially concerned with the specific, microscopic details of matter. A term that is sometimes used to classify the characteristics of an object at this level is material. For example, in a chemistry laboratory, the laboratory bench may be constructed of the material wood, the flask may be constructed of the material glass, and the safety goggles may be constructed of the material plastic.